The incidence of post-traumatic stress disorder (PTSD) among combat-experienced Veterans is ~20%, which is substantially larger than in the general population (~3.5%). Moreover, 40-50% of Veterans suffering PTSD are also diagnosed with substance use disorders (SUDs). Patients with comorbid PTSD and SUDs have greater drug use severity and show poorer treatment outcomes than patients diagnosed with either PTSD or SUDs alone. This special need of returning combat Veterans is largely unaddressed by preclinical research. PTSD and SUDs share in common the DSM-V characteristic that environmental stimuli associated with a stressor or drug use can precipitate symptoms of the disorder. Conditioned drug cravings involve activation of a circuit containing the prefrontal cortex and nucleus accumbens, and repeated drug use produces enduring changes in synaptic plasticity in the accumbens. Also, drug-conditioned cues elicit drug seeking in animal models of relapse by inducing transient synaptic plasticity at these synapses. We recently published and present further new data that a single episode of acute restraint stress in rats produces long-lasting (>3 weeks) changes in accumbens synapses that parallel the changes produced by addictive drugs. The overarching hypothesis in our proposal is that cues predicting stress or drug delivery employ the same cortico-accumbens mechanisms to elicit drug seeking and conditioned stress responding, and that these mechanisms underlie comorbid PTSD and SUDs. Cannabis is among the addictive drugs most widely abused by Veterans. We recently developed a model of cannabis self-administration and cue-induced drug seeking in rats that uses a combination of two constituents of cannabis, ?9-tetrahydrocannabinol (THC) and cannabidiol (CBD). We propose to use THC+CBD self- administration and reinstated drug seeking in combination with acute restraint stress to evaluate how cannabis use and conditioned stress interact through accumbens synaptic plasticity to promote stress-induced drug seeking. Our investigation will utilize recent discoveries showing that quantifying synaptic changes in the canonical pre- and postsynapse is insufficient to understand the transient plasticity produced by drug cues. Accordingly, we will also quantify signaling in the protein-rich extracellular matrix (ECM) that surrounds the synapse, and changes in perisynaptic astroglial processes that regulate synaptic transmission through the patterned expression of proteins adjacent to the synaptic cleft. Together, these four synaptic compartments are referred to as the tetrapartite synapse. The three proposed Specific Aims will be sequentially engaged. Aim 1 characterizes the behavioral and synaptic effects of conditioned stress using the defensive burying model of stress responding that will allow correlations to be evaluated between stress responding and measures of tetrapartite synaptic plasticity. Aim 2 uses the information garnered in Aim 1 to investigate the interactions between conditioned stress and THC+CBD use and seeking. Conditioned stress will be used to reinstate THC+CBD seeking and we will compare the intensity of drug seeking with measures of tetrapartite synaptic plasticity. Finally, in Aim 3 we endeavor to prevent the interactions between conditioned stress and cannabis use by manipulating key proteins regulating tetrapartite synaptic plasticity, including the astroglial glutamate transporter (GLT-1) and specific matrix metalloproteases that catalytically signal synaptic plasticity. Through completion of these Specific Aims, we expect to identify overlapping brain circuitry and cellular mechanisms between conditioned stress and cannabis seeking that can be explored in future studies as sites of pharmacotherapeutic intervention for treating the high incidence of PTSD and comorbid SUDs in our returning combat Veterans.